An image sensor may be a semiconductor device that converts optical images into electrical signals. Image sensors may be largely classified as a charge coupled device (CCD) image sensor or a CMOS (Complementary Metal Oxide Silicon) image sensor (CIS).
A CMOS image sensor may have photodiodes and MOS transistors formed in unit pixels, which may sequentially detect electrical signals of each unit pixel by a switching method. This may result in realizing an image. A CMOS image sensor may have a structure in which photodiodes may be arranged horizontally with transistors. Even though some lateral CMOS image sensors may overcome certain limitations of CCD image sensors, they may still have various disadvantages. For example, in a lateral CMOS image sensor, a photodiode and a transistor may be fabricated next to each other on and/or over a substrate. Thus, additional area may be required for the photodiode. This may reduce a fill factor area and may also limit a resolution. In addition, it may be difficult to optimize a process technology for preparing both the transistor and the photodiode at the same time. For example, a fast transistor process may require a shallow junction for low sheet resistance, but such a shallow junction may be unsuitable for a photodiode process. Moreover, according to a lateral CMOS image sensor, additional on-chip functions may be added to an image sensor. Hence, either a pixel size may need to be increased to maintain a sensitivity of the image sensor, or an area required for a photodiode may need to be decreased to maintain a pixel size. However, if a pixel size increases, a resolution of the image sensor may decrease, and if a photodiode area decreases, a sensitivity of the image sensor may decrease.